1. Field of the Invention
The present invention relates to telephone test sets.
2. Description of the Prior Art
FIG. 1 illustrates a prior art telephone test set which has been manufactured by the assignee of the present invention as the Dracon Model TS 21-080 test set.
A telephone test set is commonly used in two distinct modes of operation. The first mode of operation involves the operation on normal telephone lines which involve the establishment of talking circuits between the test set and the telephone central office which typically have 5 volts or more voltage drop within the test set. The second mode of operation involves the so-called "dry loop" wherein a pair of test sets are connected to the opposite ends of a pair of wires to enable the linepersons to identify the ends of a single pair of wires. This dry loop mode of operation does not receive battery voltage from the central office and functions with the pair of test sets sharing 3 volts or less provided by a small battery minus line losses. There is considerable resistance by the various telephone operating companies to the purchase of test sets which can only operate in a single one of the two modes of operation described, supra.
Prior art telephone test sets which can perform in both of the above-described operating modes typically include a two-position switch which is commonly known as a "keypad-in/out" switch. The switch permits choice between the mode of operation requiring signaling capability and the mode which does not require signaling capability. The prior art test set of FIG. 1 has a three pole two position switch, having contacts S1A, S1B and S1C, used to determine the mode of operation under which the test set operates. In the position labelled "out", the speech network is connected directly to the tip and ring lines. The contact S1C prevents the lineperson from being subjected to loud bursts caused by inadvertent contact with the keypad of the dial pulse switch. The contact S2 is used in its closed position for conducting a polarity test of the tip and ring lines to check the direction of current flow between the tip and ring lines by the activation of the oppositely poled light emitting diodes DS1 or DS2 which respectively emit light of a different color when current flows through each diode to inform the lineperson of the direction of current flow.
With the prior art circuit of FIG. 1 being operated in the "keypad in" mode, it is necessary to connect the speech mute switch in series with the speech network. In this configuration, the mute switch must function regardless of the direction of current passing through the test set and so polarity guard BR is necessary. Unfortunately, the non-linear voltage drop of the polarity guard BR can cause the speech network to become inoperative at extremely low tip to ring voltages which are present when the telephone test set is used for conducting line tests with dry loops which are not powered by a central office battery. Moreover, voltage drops in the dial pulse switch can also severely affect the operation of the speech network in low voltage conditions. Finally, with low voltage conditions aggravated by the series connection of the dial pulse switch, speech mute switch and speech network with the polarity guard bridge, the total voltage drop within the test set is such that the insertion of any additional series connected elements would result in excessive total voltage drop; and so a continuously indicating series connected polarity indicating means is not feasible in this prior art. Thus, the polarity indicating circuit is selectively coupled across the tip and ring lines by the closure of switch S2 to permit only intermittent polarity testing.
The three pole two position switches Sl and S2 are sealed switches which are expensive and difficult to obtain, are a leading cause of test set in the field failures, make the operation of the test set cumbersome to the linepersons and make it necessary for the design of the housing for the test set to have a number of openings where moisture may enter.
U.S. Pat. No. 3,708,634 discloses a telephone test set in which a voice receiver unit is partially coupled across the tip and ring lines. The voice transmitter unit is connected in series with a polarity guarding bridge. A SCR is connected in series with the polarity guarding bridge and the voice transmitting unit. The SCR is biased into conduction by the manual closing of a momentary contact switch which then provides a conduction path for the operation of the voice transmitter unit. The SCR is turned on when the voice transmitter unit and voice receiving unit are operational.
The system disclosed in U.S. Pat. No. 3,708,634 is not well suited for operation on dry loops which have low operating potential because of the fact that the voice transmitting unit is connected in series with the polarity guard bridge.
The use of a thermistor in series with the lead of a telephone test set which is adapted to be connected to the tip or ring lines of a subscriber's telephone line is known to protect the telephone test against high current conditions on the subscriber's telephone line. Moreover, the use of a silicon controlled rectifier as a protective mechanism to a hold circuit in a telephone test set is known.
A telephone test set in accordance with the present invention has advantages which are not achieved by the prior art telephone test sets. The telephone test set of the present invention may be used to activate the voice transmitting and receiving circuits when connected to a dry loop which does not receive operating potential from the central telephone office. The transmitting and receiving voice units are connected directly across the leads which are adapted to be connected to the tip and ring lines which removes the effect of the voltage drop in the polarity guard from influencing the operation of the transmitting and receiving units. The elimination of the voltage drop caused by the polarity guard makes it possible to connect an indicator for the direction of current flow between the tip and ring lines directly in series with the voice transmitting and receiving units to permit a continuous indication of the direction of current flow which is useful to the lineperson.
A telephone test set in accordance with the present invention includes first and second leads which are respectively adapted to be connected to a tip line and a ring line of a subscriber's telephone line; a full wave rectifier for providing a rectified output of an input signal and having a pair of input terminals which are respectively connected to the first lead and the second lead and a pair of output terminals which respectively output a voltage of a first polarity and a second polarity; a shunt circuit coupled between the pair of input terminals of the full wave rectifier which is comprised of a series connection of a transmitter and receiver of voice communications and a switch having an open circuit condition when a control signal of a first magnitude is applied to a control terminal and having a closed circuit condition when the control signal is applied to the control terminal of a second magnitude to permit current to flow within the shunt circuit; a signaling circuit for placing a telephone call over the tip and ring lines, coupled between the pair of bridge rectifier output terminals, for producing signals to place a telephone call over the tip and ring lines to a central office when in an activated state and for not producing signals when in an inactive state and a control circuit responsive to the signaling circuit for producing the control signal having the first magnitude when the signaling circuit for placing a telephone call is in the active state and for producing the control signal of the second magnitude when the signaling circuit for placing a telephone call is in the inactive state.
The shunt circuit may include an indicator for continuously indicating the direction of current flow between the tip and ring lines of the subscriber's line which is connected in series with the transmitter and receiver of voice communications and the switch. The switch may be comprised of a pair of field effect transistors each having connected source located within the path of current flow in the shunt circuit with the gates of each of the field effect transistors coupled to the signaling circuit. The control of the field effect transistors may be made by an optical isolator which is activated by the signaling circuit which causes the field effect transistors to be turned off when the signaling circuit is activated.
The indicator for continuously indicating the direction of current flow between the tip and ring lines comprises a pair of oppositely poled light emitting diodes which respectively emit light of a different color. An audio frequency bypass, which shunts the oppositely poled light emitting diodes, permits audio frequency communications to be coupled to the subscribers' telephone lines without attenuation by the diodes. A resistance is provided, which shunts the oppositely poled light emitting diodes, to permit conduction of current in the shunt circuit when a voltage across the light emitting diodes is insufficient to bias one of the diodes substantially into conduction. A voltage limiter is provided which shunts the pair of light emitting diodes to limit the voltage which is applied to the diodes to a level which will not cause their failure. The voltage limiter may be plurality of serially connected diodes located within a bridge.
The signaling circuits are protected by a voltage limiter to prevent excessive voltages being applied which is comprised of a second shunt circuit coupled between the output terminals which shunts the output terminals of the full wave rectifier and the signaling circuits. The second shunt circuit includes a variable impedance which shunts the output terminals with an impedance which decreases nonlinearly as an applied potential increases in magnitude. Preferably the variable impedance is a metal oxide varistor. Furthermore, a switching circuit is coupled between the output terminals which is biased into conduction by the application of a control signal of a first magnitude to cause the output terminals to be short circuited together and to have an open circuit condition when the control signal is below the first magnitude. The control signal is produced by a RC circuit coupled to the variable impedance and to one of the output terminals. Preferably, the switching circuit is a SCR having an anode and a cathode coupled respectively to the output terminals and a gate coupled to the RC circuit. A voltage limiting circuit may be coupled to the variable impedance and one of the output terminals with the gate of the SCR being coupled to the voltage limiting circuit to protect the gate against excessive voltage.
An indicating circuit within a telephone set for continuously signaling the direction of current flow between the tip and ring lines of a telephone line in which the indicating circuit is connected in series with a transmitter and receiver of voice communications within the test set includes a pair of oppositely poled light emitting diodes for respectively indicating different directions of current flow between the tip and ring lines by emitting light of a different color, the pair of oppositely poled diodes being coupled between one of the lines of the telephone line and the transmitter and receiver of voice communications; an audio frequency bypass which shunts the oppositely poled light emitting diodes to permit audio frequencies to bypass the light emitting diodes; and a resistance which shunts the oppositely poled light emitting diodes to permit conduction of current in the shunt circuit when a voltage across the oppositely poled light emitting diodes is insufficient to bias one of the light emitting diodes substantially into conduction. A voltage limiter may be provided which shunts the pair of light emitting diodes to limit the voltage which is applied to the diodes. Preferably, the voltage limiter is a plurality of series connected diodes which are configured in a bridge.